An image sensor that is installed in a tip portion of an insertion unit of an endoscope and an imaging optical system for forming a subject image on the photodetecting surface of the image sensor are typically disposed in the tip portion of the insertion unit in such a manner as to be housed in a holding frame. The insertion unit of the endoscope is formed with a treatment tool channel in which various treatment tools for treating an observation part can be inserted, and the treatment tool channel and the holding frame which houses the image sensor and the imaging optical system are disposed in the tip portion of the insertion unit adjacent to each other in a radial direction.
JP-A-2015-58118 and JP-A-2015-73537 disclose endoscopes in which a bottom wall, extending alongside a treatment tool channel, of a holding frame that holds an image sensor and an imaging optical system is so thin as to reduce the diameter of the insertion unit having a tip portion in which the holding frame and the treatment tool channel are arranged adjacent to each other in a radial direction.